You can add a secondary IP address on any interface (e.g.) without using VLAN tagging. This is useful if you have more than one subnet on your LAN, have no VLANs but want both subnets to be routable, without using an additional physical interface.

Create a static ARP entry for the gateway address of the additional subnet on the interface. Select the interface on which the subnet will be on. Enable Publish ARP. This will also automatically fill in the MAC address of the selected interface.